The dimensions within silicon integrated circuits, particularly field effect devices, continue to grow smaller. One of these dimensions is the thickness of the gate oxide layer. For acceptable yield of devices during manufacturing this thickness needs to be controlled to better than about 10%. In TABLE I below we have summarized the required thickness (and acceptable deviations therefrom) as a function of the critical dimension of the devices as well as the voltage that will be applied across these oxide thicknesses.
TABLE I thickness/voltage 5V 3.3V 2.5V ??V 0.6 .mu.m 125 .+-. 10.ANG. 0.5 .mu.m 115 .+-. 10.ANG. 85 .+-. 10.ANG. 0.35 .mu.m 110 .+-. 10.ANG. 70 .+-. 10.ANG. 0.25 .mu.m 48 .+-. 3.ANG. 0.18 .mu.m 25-40 .+-. ?.ANG.
As can be seen, gate oxide thicknesses less than 50 Angstroms will be required in the near future with control of these thicknesses to better than about three Angstroms. The thickness of transparent films in this range is most easily measured by means of ellipsometry. Ellipsometry is based on measuring changes in the state of polarization of light reflected from a substrate. The state of polarization is determined by the relative amplitude of the parallel and perpendicular components of the radiation as well as the phase difference between them. On reflection from a surface, whether it is bare or covered by a transparent film, the ratio of the amplitudes and phase differences undergo changes that are dependent upon the optical constants of the substrate, the angle of incidence, the optical constants of the film, and the film thickness. Thus, if all the other relevant quantities are known, the thickness can be determined to a high level of accuracy from such measurements
When a large number of measurements of layers of the same material, having about the same thickness, are to be made, as in a production environment, calculating each thickness value from scratch can be very time consuming. It is usually more convenient to make measurements that relate to a standard film whose thickness as well as its refractive index is accurately known. While such reference standards are widely used, it has been found that over an extended period standards of this type tend to drift in terms of their thickness value. In addition to a steady thickness growth of about 0.2 Angstroms per day, it is also found that the thickness value fluctuates on a day-to-day basis in an unpredictable manner.
In FIG. 1 we show a plot of thickness as a function of time over a period of approximately one month for several wafers that were used as thickness standards of the prior art design. The measured standard deviations for these curves are shown in TABLE II below:
TABLE II Standard CURVE # Deviation 11 0.1579 12 0.2961 13 0.1698 14 0.2855 15 0.2056
It has been suggested (by the Rudolph Corp., a manufacturer of ellipsometric equipment) that the changes in measured thickness are mainly due to contamination of the exposed surface (as opposed to continued growth of the oxide proper). They confirm this by their finding that careful cleaning of a wafer standard whose thickness value has drifted returns the thickness once again to its original value. This is illustrated in FIG. 2 where a film standard whose value drifted gradually from about 14.5 Angstroms to about 18 Angstroms over a period of about 180 days was restored to its original value by washing in hydrogen peroxide. At the end of an additional 130 days the thickness value had drifted once again but, as before, could be restored to its original value by using the same cleaning procedure.
While the cleaning procedure described by the Rudolph Corp. clearly works, in a manufacturing environment, the standard wafers would need to be cleaned on a daily basis and possibly even several times a day. Such a procedure is time-consuming with the potential of holding up the production line. Therefore a thickness standard that is stable and does not require regular cleaning is to be preferred. The present invention discloses how such a standard may be formed.
No prior art anticipating the present invention was discovered during our routine search. However, several references of interest were found. Dagenais et al. (U.S. Pat. No. 5,354,575) teach the use of ellipsometry for measurement of anti-reflection coatings used in laser amplifiers but do not discuss the issue of providing a stable standard. Nagy et al. (U.S. Pat. No. 4,717,446) teach a method to measure a film thickness using a monitor wafer while Saxona et al. (U.S. Pat. No. 5,751,582) show a site model and monitor wafer controller for an oxidation process.